This study will determine whether greater heat storage during exercise after simulated spaceflight is due to impaired skin vasodilation or impaired sweating. Before & immediately after 13 days of bed rest, 8 subjects will perform a submaximal, supine exercise test with measurements of core temperature, skin temperatures, skin perfusion, and local chest sweating. Changes in thermal responses will be related to changes in blood volume during the bedrest, as measured via isotope dilution.